Extremely low frequency (ELF) electromagnetic waves have frequencies between 1 and 30 Hz. ELF waves are found as a natural occurrence in the environment and in man-made electronic devices. Naturally occurring ELF waves resonant in a cavity formed by the earth's surface and the ionosphere. The conducting terrestrial surface of the earth and the conducting ionosphere separated by an insulating layer of air creates the earth-ionosphere cavity. The ELF resonance in the earth-ionosphere cavity is generally referred to as Schumann resonance and can be viewed as separate spectral peaks.
Many sensors used to measure Schumann resonances consist of two horizontal magnetic induction coils for measuring the north-south and east-west components of the magnetic field and a vertical electric dipole antenna for measuring the vertical component of the electric field. However, the ELF frequency range generally overlaps the vibration spectrum of the antenna, which induces unwanted noise voltage in the antenna. To stabilize the antenna, the length of the antenna is generally increased. For example, the vertical component of the electric field can be measured using a 36.5 meter vertical mast. Other antennas used to measure the ELF resonance include extremely large horizontal antennas, ball-over-plane configurations having an antenna height of approximately 1 meter, and spherical antennas mounted on a gimbaled mast approximately 10 meters above the ground.
Smaller entities, such as organs and limbs of the human body also produce ELF resonant cavities. Each organ and limb resonates in a different frequency. However, the antennas previously developed are too large for conveniently measuring ELF resonance in the human body.